Insects at sea

Insects are primarily terrestrial. While a majority of insects are adapted to different habitats on land, a small number have secondarily taken to life in water. A typical insect respires through tracheae, which are branching tubes carrying air deep into the body to various tissues. Most aquatic insects also respire with help of tracheae along with some special features to carry undissolved air close to tissue fluid. These special features, one or more of which may be present in an aquatic species, include: (i) A closing mechanism for spiracles, which are little windows, through which the tracheae communicate with the atmospheric air. The closing mechanism prevents entry of water into the tracheae, when the insect is submerged, and opens the spiracles, when the insect rises to the water surface for breathing the atmospheric air. (ii) An air store on the body surface. The spiracles open into the air store, and thus air breathing continues even during submergence. The air store may be renewed during visit to the water surface. (iii) Some aquatic insects possess tracheal gills, which are folds of thin skin, with a rich network of tracheae within them. Diffusion of dissolved oxygen from the surrounding water into the tracheae provides the necessary requirement of undissolved oxygen reaching deep into tissues. A plastron, which is a dense pile of hydrofuge hairs holding a semipermanent thin film of atmospheric air, is used by some beetles in fresh and saline water. Some small aquatic insects have blood gills, which are folds of thin integument full of blood. Direct diffusion of dissolved oxygen from the surrounding water into blood serves for respiration.

These details about insect respiration attest the statement that insects are primarily terrestrial, as almost all of them need undissolved air in their tracheae for respiration.

Though insects are primarily terrestrial, some of them have become well adapted to life in water. But surprisingly most aquatic insects have chosen fresh water environs, and very few have taken to life in sea. Actually 5% of all insect species are aquatic in rivers and lakes, whereas fresh waters constitute 0.01% of the total amount of water in the biosphere, and the oceans cover 71% of the Earth. It is evident that competition was the main driving force in evolution of land arthropods, including insects, in taking to aquatic life only in small numbers, as Crustacea and Trilobites had occupied in the past most of the available niches in water, and fishes were there as terrific predators. Arthropods, like Limulus, survived in water, thanks to a strong armored body resistant to any fish attack. Their spiny rounded cephalothorax is probably an obstacle to swallowing by fishes.

Mackerras (1950) has reviewed marine insects, but his paper is very brief. A recent general review of marine insects has been done by Lanna Cheng (in Resh and Cardé, 2003). Among the 15 or so orders of Insects, living in marine or near marine habitats, the most important species are found in Collembola, Heteroptera, Homoptera, Coleoptera and Diptera. Lice, found among sea mammals or birds, present often some adaptation to sea water. Cheng (1976) distinguishes for sea dwelling insects 5 habitat catégories: pelagic, coastal, intertidal, mangrove and saltmarsh. Water can be brackish in mangrove areas, which in Thailand harbour even a frog, whereas Amphibia in general shun saltish water. Insects are common in all those habitats, except in the open sea.

There are always exceptions in biology and you cannot readily generalize when dealing with living beings. To bring home this point let us see some examples among insects found in habitats other than sea. Among freshwater insects, at the larval stage, like Odonata, there is a remarkable exception in Hawaii, where is a dragon fly ('Megalagrion oahuense), which has a terrestrial larva. The eggs are laid among trash under thickets of a fern, Gkichenia linearis. The nymphs live in the damp trash in the mountains of Oahu and are densely hairy (Zimmerman, 1948). Some other dragon fly larvae are arboreal, but they live in phytotelmata. Larvae or nymphs of Odonata or dragon flies are as a rule fresh water forms. No dragonfly has taken to the sea. Caddis flies (Trichoptera) are normally with aquatic larvae, but the genus Enoicycla is unique in Europe, having flightless females and terrestrial immature stages. The larva is a typical detritivore and is a typical lim-nephilid, with the exception of gills, which are absent. The female lays about 50 eggs among mosses at the base of a tree. After hatching, the young larvae construct conical cases, mainly from organic matter and start feeding on mosses, algae and tree-leaf litter (Harding, 1995). In North Africa, there is even another limnephilid, related to Enoicycla, Enoicykpsis peyerimhoffi, which lives in dry forest surroundings (Masselot and Dortel, 2004).

If we talk of terrestrial arthropods other than insects, no scorpion has come back to the sea, from where they originated. Many terrestrial spiders, mites and insects (from Orthoptera to Diptera and Coleoptera) are adapted to sea life, at least in the tidal zone, along the shores, in the rock crevices, and also mites and collembola in rock pools. Some caddis-flies (Trichoptera) around Australia, New Zealand and New Guinea, lay eggs into starfishes and the larva is a tube case maker in the echinoderms (Neboiss, 1988). In Australia there are even parasitoids among larvae of Trichoptera (Wells, 1992), but all in fresh water. That has been a recent finding. Remarkably, no mosquito (Aedes, Culex, Anopheles) larvae live in the open sea, but can withstand a very high salinity in rock pools, streams or lagoons. Perhaps they could not survive in sea because of predators. Chironomid larvae are found in salty marshes with a salinity heavier than in the sea, but there are no predators to worry about. Among the bugs, Corixidae can breed in ponds with a salinity approaching saturation. Unlike Halobates (vide infra), corixids are winged and migrate by flight. An old paper by Buxton (1926) describes the colonization of the sea by Pontomyia natans, a chironomid midge in the Samoa. It's a lagoon frequenting species, and it is the only known insect which is submarine in all stages. The male swims actively through the water, using its long first and third legs. Pontomyia has an extremely short adult life (30 min to 3 hours) and the pupae float to the sea surface. Of this genus only 4 species are known. Chironomidae, Dolichopodidae and Tipulidae are often associated with intertidal algal turf (Resh and Cardé, 2003).

Many beetles (Staphylinidae, Carabidae, Curculionidae) are found, along with Hemiptera (Veliidae, Hermatobatidae, and others), in rock crevices or in intertidal areas, on the sea shore. They are submerged at high tide like the European Aepus robini and the related species. Bkdius spectabilis, a staphylinid beetle, maintains a burrow that prevents flooding, and provisions the young with algae, prevents mold and protects its larvae from parasitoid attacks (Pelissier Scott, in Resh and Cardé, 2003). In Australia, Britton (1971) reported a Limnichidae, Hyphalus insularis, from the intertidal zone together with bugs, and also melyrid and staphylinid beetles. The whole body is covered with a silvery film of air. The genus occurs in interstices of intertidal coral slabs on the Great Barrier Reef. It has been reported from Australia, Cocos, Howe, Norfolk, Japan and New-Zealand. It should exist also in New Caledonia and New Guinea where it has never been searched for. Hyphalus larvae have anal gills, a type of blood gills, for respiration (Lawrence and Britton, 1994). Recently, Hernando & Ribera (2004) found the second species of the genus Hyphalus from the Indian Ocean (Seychelles). The first one was found in Aldabra. Similar beetles exist all over the world on the submerged part of the sea shore, with spiders, mites, bugs (A^epophilus) and various other insects. Many beetles frequent the sea, and one chrysomelid, Macroplea mutica, a donaciine, is entirely marine in the Baltic sea, and it feeds on Zostera. One dermapteran, Anisolabis littorea, in New Zealand lives in brackish-water sponges (Cheng, in Resh and Cardé, 2003), along with other insect larvae. The gill chambers and eggs masses of marine crustaceans are poorly known habitats for immature insects (Humes, 1948). Larvae of Diptera and Coleoptera have been found on crabs in various places in the tropics. Probably, those larvae feed upon detritus and mucus in the gill chambers, but they may be capable of piercing the gill surfaces. Some chironomids seem to live naturally in the gill chambers. A Luáola firefly lives on old coral reefs near Madang, in New Guinea (Lloyd, 1973). The entire lifecycle is spent on the reefs.

Let us briefly talk about plants in the sea. Marine angiosperms or flowering plants are rare in the sea (only around 30). They are all monocotyledons, like Zostera, and van der Hage (1996) believes that their rarity is due to their pollination mechanism and the absence of coevolu-tion with insects. If angiosperms had invaded the seas, perhaps, she says, the insects would have followed. Perhaps relative absence of flowering plants in sea is because production of a large number of gametes is very costly to the plants, and pollination causes serious problems in the sea. Insect evolution predates that of the angiosperms by some 200 million years, and the reasons why the sea was not colonized is not very clear. The association of insects with green plants on land started very early and probably algae were not very attractive to them.

A marine caddis fly, Philanisus plebeius (Trichoptera), on the coasts of Australia and New zealand, oviposits into a starfish, Patiriella exigua, and the larva constructs its tube from coralline sea-weeds, in inter-tidal rock pools. It feeds on bryozoans, copepods, and other small rock-pool animals (Anderson et al, 1976). The adult female has strong ovipositor and this ovipositor is employed to insert the eggs into the coelomic cavity of the starfish where they hatch and develop into larvae. The larvae escape rapidly from the strarfish host and very probably they eat their way out through its body wall. other case-making larvae, presumably with the same biology, are known on the coasts of the south-western Pacific Ocean. Species of Philanisus, Chathamia and probably more caddis fly genera remain to be discovered and studied (Riek, 1976). The above described mode of oviposition offers protection to the caddis embryos in the intertidal habitat (Anderson and Lawson-Kerr, 1977).

There is only one insect genus, which lives in open sea; it is the water strider, Halobates, a Gerridae; 42 species of this genus are found in sea, but out of these only five species occur in open seas, while 37 species are confined to coastal waters (Pathak et al, 1998). Halobates species aggregate into flotillas on the sea water surface, like the bug Gerris, the beetle Gyrinus or the mosquito larvae in fresh water. It could be a strategy for protection, and possibly a feeding strategy for the Hemiptera and Coleoptera in fresh waters, but the group effect (Grasse) in the flotilla has never been completely understood. Water striders (Gerridae) walk on water. They have non-wetting legs that enable them to stand effortlessly and move quickly on water (Gao and Jiang, 2004). The legs are covered by large numbers of regularly oriented tiny hairs (microsetae) with fine nanogrooves, which enhance water resistance. This holds both for fresh and for the sea gerrids. only the four long hind legs are used for locomotion, while the two small front legs hold the preys to facilitate the sucking of their juices.

The oceanic species of Halobates occur in tropical and subtropical seas, often hundreds of kilometers away from any land. They are specially numerous in areas covered with sea weeds. They are seen walking or skating on sea surface.

Being wingless they must be drifting with water currents to achieve their dispersal. Habbaies are chiefly indo-pacific, and they are missing in the Mediterranean Sea, and known there only as fossils from the Eocene. Probably the drying of the sea during the Tertiary in the Mediterranean region killed them, as it killed the Merostomata. Halobaies robustus occurs on the surface of the coastal waters of the Galapagos Archipelago (Foster and Treherne, 1980), close to mangrove and lava edges. Its food consists of dead insects floating on the sea. Predation by fish, birds and reptiles (the marine iguana) is reduced by extremely effective avoidance behaviour by the flotillas. One of us (PJ) remembers the Halobaies in the Red Sea, on the Ethiopian coast, along the small mangroves of those islands, going often far away into the open sea. Food was rare in those semi-desert environments, as the islands are practically bare, but for halophytic plants.

Members of Gerridae, the bug family to which Halobaies belongs, are almost confined to fresh water bodies, and are seen skating about where water is stagnant and quiet. Their middle and hind legs are very long, and the tarsi of the legs are covered with long branching hairs, which are difficult to wet. These hairs spread out on water surface, and this makes possible for the insect to skate around on the surface of water. If somehow the tarsi go wet, the insect will sink. In this situation the water strider has to climb on some solid surface and expose itself to air for sometime to dry its tarsi, so that it may skate around again on water. Front legs of a water strider are quite short and foldable in such a way that they may hold certain floating objects, which are generally dead insects, which fall to water surface and constitute the main source of nourishment for the water striders. It is surprising that out of the huge class of insects some members of only one family, Gerridae, most members of which live on placid waters, have become adapted so well to marine life.

As has been pointed out above, most sea dwelling species of Halobaies are confined to coastal waters. Water striders of a related family, Veliidae, and some other bugs also occur in coastal waters. In fact a number of land or shore living insects may venture or fall into coastal waters, but they are destined to perish, unless they swim back to coast.

Though truly marine habit is confined to a few species of the surface strider Halobaies, many other insects are met with in sea. Terrestrial insects, mostly small and light bodied ones, rise in the atmosphere with thermal air columns and may be blown out into sea for long distances by air currents or winds. This way the insects may drift over sea for hundreds or even thousands of miles. Such air drifting insects have been trapped and studied over the Pacific, the Antarctic and some other oceans by several workers, mainly by Gressitt, Cheng and their associates (Cheng and Birch, 1977 and 1978; Gressitt et al. 1960 and 1961). More recently insect trapping over the Bay of Bengal and the Arabian Sea and their study have been carried out by Pathak and his team (Pathak et al, 1999a and b). The collections made over the Indian Ocean mostly included small beetles, flies, bugs and wasps. These air borne terrestrial insects eventually fall dead to the sea surface. They may be collected floating on sea water. They constitute a source of nourishment for marine life, including the marine Halobates (Jolivet, 1991).

We may find terrestrial insects at sea under another situation. Dragonflies are known to fly following moving objects for reasons not known. Pathak (1996) noticed several dragon flies following and flying over an oceanography research vessel right from Marmagoa Port in the western coast of India to the Lakshadweep group of Islands. They perhaps periodically rested on structures on the ship. It was not a dragonfly in any way adapted to sea life. Pathak et al. (1988) noted a butterfly alternately resting on the upper deck of the ship and flying following a loop like course over the sea surface to return to the ship.

Thus the oceans, not inhabitable for insects in general, are not free from them.